The objectives of this proposal are to provide an understanding of the mechanism(s) of regulation of the amounts of the primary bile acids, cholic and chenodeoxycholic acids, formed from cholesterol in the liver, and to delineate the details of their biosynthetic pathways. Biochemical approaches include a study of optimal molecular configuration of the substrate required for hepatic microsomal 12alpha-steroid hydroxylase with appropriate 5beta- and 5alpha-sterols, a study of the separation and purification of this microsomal enzyme, a study of mechanisms involved in shortening the side chain of the C27 derivative to a C24 bile acid, a study of the effects of biological agents on the synthesis and control of C24 bile acids, and a study to ascertain whether plant sterols contribute to the bile acid pool. Chemical syntheses of needed intermediates will be undertaken, with analyses carried out by gas chromatography-mass spectrometry, coupled with a data acquisition system, high performance liquid chromatography, glass capillary gas chromatography, nuclear magnetic resonance spectroscopy, and other physical methods.